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PHOTO ID REQUIRED! Midterm 1 • 30-35 multiple choice questions. • About material covered in lectures. SIT IN YOUR ASSIGNED ROW! A seating chart will be displayed on the screen. • Study: – – – – – Notes. Weekly review questions. Study guide – will post on Friday. Parts of textbook that I talked about in class. (sample questions on course web site… briefly) • Closed book, closed notes. • No calculators or cell phones. • Office hours (BPS room 3270): – Mon 12:45-2:00 – Tue 3:00-4:00 – Or by appointment Midterm 1 study guide – page 1 Exam will cover everything that was in the lectures, not just what is in this study guide. The lectures covered: • Units, etc used in astronomy; relative size scales. • History of astronomy from ancient Greeks until mid-1800’s. – What did the ancient Greeks have right? What did they have wrong? – How we went from geocentric (Earth at center) models to heliocentric (Sun at center) models of Solar System. – Ptolemy’s system – epicycles, etc. Why were these complications needed? – Retrograde motion. – Copernican revolution. – Galileo's observations. • The nature of science. • How things move – Kepler’s 3 laws – Newton’s 3 laws + law of gravity – Conservation of energy, angular momentum • “Conservation” means that these quantities stay constant unless something is done to the system from the outside.. • Be able to use the concept to reason your way through what will happen in simple situations such as those described in class. – Escape velocity & orbits 1 Midterm 1 study guide – page 2 The lectures covered (continued): • Electromagnetism & Light – The electromagnetic wave – Dual wave/particle nature of light • What is meant by this? – The electromagnetic spectrum • Different names for light at different wavelengths • Measuring the spectrum of an object [Fig. 5.9] Energy – Emission & absorption lines • What are they? • How do we interpret them in terms of energy level diagrams? – What can they tell us about the gas that does the absorbing or emitting? Midterm 1 study guide – page 3 [Fig 5.11] The lectures covered (continued): – Continuous radiation • What is it due to? • How can it tell us the temperature of the emitting object? – Under what circumstances do we see emission lines, absorption lines, continuous radiation? – Doppler Effect • What is it caused by? • What is a redshift? What is a blueshift? Wavelength Frequency Energy – Telescopes (but only if I actually talk about them on the Monday before the exam) • Visible light telescopes • Radio telescopes • Telescopes in space. Why put them there? See [Fig 5.8] 2 Formation of spectral lines Emission spectrum: Planetary nebula shell. Gas cloud [Fig 5.8] Everybody else sees emission lines, but this observer sees absorption lines. Light source Emission-Line Spectrum • Collisions or absorbed light moves electrons to higher energy levels. • Electrons then fall to lower energy levels. Emission Lines Absorption-Line Spectrum Atoms in gas cloud remove photons that have correct energy to move electrons between energy levels. Gas Cloud Absorption Lines Stars: absorption lines Hot interior • Inner layers = hot light source. • Outer layers = cooler gas cloud. Wavelength Some “laws” to know Kepler’s laws [pgs. 67-68]: 1. 2. Each planet moves around orbit in ellipse, with sun at one focus. The straight line joining the planet and the sun sweeps out equal areas of space in equal amounts of time. What does this tell us about speeds of planets in different parts of their orbits? 3. P2 = a3 • • P = period of orbit, in years a = semi-major axis of orbit, in au. What does this say about planets’ orbital periods and average speeds in their orbits? Newton’s Laws of Motion [pgs. 91-92]: 1. In the absence of a net (overall) force acting upon it, an object moves with constant velocity. 2. Force = mass acceleration 3. For any force, there is an equal and opposite reaction force. …and Newton’s law of Gravity [pg. 98]: Fgravity Gm1m2 r2 3 Some formulae to know (these will NOT be provided to you on the exam) Motion: Newton’s 2nd Law: Newton’s law of Gravity: F = ma Fgravity Gm1m2 r2 = ½ mv2 = mvr Kinetic energy Angular momentum (F = force) (r = separation between the two objects) (here r means distance from spin axis) Light: f = frequency Frequency f, wavelength : f = c/ Energy of photon: E = hf = hc/ Thermal emission per unit surface area = const.× T4 c = speed of light. = wavelength h = Planck’s constant const. = other constants T = temperature You don’t need to know the values of any of the physical constants. But you should know the formula, and be able to use it to solve simple proportionality problems: For example: if I double the force acting on an object, how many times bigger or smaller is its acceleration? And similar questions involving the other equations.. Special Relativity [pg. 364] Einstein postulated (1905): • The Principal of Relativity. The laws of physics are the same in all inertial reference frames. • The constancy of the speed of light. Light travels through a vacuum at a speed c which is independent of the light source. distance, time, velocity add up in funny ways Note regarding the Midterm: You don’t need to know these equations or how to use them. But I include them here to illustrate what the words mean. u = 70 mph v = 100 mph For slow speeds: u = 70 mph v = 100 mph c = 669,600,000 mph 1 - uv/c2 = 1 - .00000000000001 v’ = (v-u)/0.99999999999999 v′ = (v-u) Classical: Special relativity: v' v u uv 1 2 c But all observers see light move at same speed: v' cu c uc 1 2 c 4